A kinetic investigation of gibbsite precipitation using in situ time resolved energy dispersive X-ray diffraction

The precipitation of gibbsite Al(OH)(3) from a supersaturated caustic alumi nate solution is the rate determining step in the production of alumina (Al 2O3) by the Bayer process. Seeded secondary nucleation and ordered growth e xperiments of gibbsite from sodium, potassium and deuterated sodium alumina te solutions, at 60 and 80 degrees C, have been investigated for the first time using the in situ energy dispersive X-ray diffraction (EDXRD) techniqu e. Gibbsite was the only phase precipitated from sodium and potassium alumi nate solutions under all experimental conditions investigated. A mixture of gibbsite and bayerite was precipitated from deuterated sodium aluminate so lution at 60 degrees C while gibbsite was the only phase observed at 80 deg rees C. Bayerite persisted over the duration of the experiment, suggesting that bayerite and gibbsite precipitated independently and that bayerite did not undergo polymorphic transformation to gibbsite. Kinetic calculations u sing the Cardew model indicate that seeded precipitation at 60 degrees C is a nucleation controlled reaction with nucleation occurring more readily in potassium aluminate solutions than in corresponding sodium aluminate solut ions. Calculations of the growth rate for experiments at 80 degrees C sugge st that growth occurred preferentially over nucleation. These calculations confirm that suitable experimental conditions were chosen for the promotion of either nucleation or ordered growth over the other growth processes fro m caustic aluminate solutions.